The subject matter of the present invention relates generally to drainage apparatus for a papermaking machine and in particular to a stationary drainage device including a drainage box closed by a stationary curved cover formed by cover elements of ceramic material and including a central cover element of greater width in the direction of travel of the paper sheet than the other cover elements on opposite sides thereof and a pressure roll which presses the paper sheet and the conveyor for such sheet against the upper surface of the central cover element.
It has previously been the practice in papermaking machines to provide a suction couch roll at the output of the forming section of the papermaking machine adjacent the porous conveyor "wire" on which the paper sheet is formed by draining water from the paper pulp stock through the conveyor. The suction couch roll is mounted at a position downstream from where the paper pulp stock is formed into a self-supporting paper sheet at a position immediately upstream from where the paper sheet is transferred from the conveyor wire to the press section of the machine onto a felt conveyor in such press section. The suction couch roll rotates and contains one or more stationary suction boxes within the roll requiring vacuum seals between the suction boxes and the inner surface of the shell of the couch roll. Also, the porous conveyor carrying the paper sheet contacts only a small portion of the couch roll in an arc of less than about 60 degrees. This reduces the effective surface area of the couch roll available for suctioning the water from the paper sheet to less than 20 percent. In addition, the couch roll is a complicated and expensive apparatus which is difficult to maintain because the seals must frequently be replaced requiring removal of the roll for maintenance. Because of this it has been proposed in U.S. Pat. No. 4,880,500 of Eldridge issued Nov. 14, 1989 to provide a stationary couch device with a curved cover in place of a rotating suction couch roll. However, this stationary couch device has several disadvantages since it is made of thick self-supported segments of ceramic material which are drilled to provide circular holes or elongated slots in such cover segments for drainage. These drilled holes or slots are subject to clogging which reduces dewatering efficiency and requires special cleaning spray channels to be provided in the ceramic cover segments making the couch device extremely expensive and somewhat impractical.
It has previously been proposed to provide a pressure roll above the rotating suction couch roll in order to press the conveyor wire and paper sheet between the press roll and the couch roll for more uniform distribution of water and to break lumps in the paper sheet. Another use of press rolls is shown in U.S. Pat. No. 3,824,152 of Nevalainen issued July 16, 1974 on a twin-wire papermaking machine with a rotating suction cylinder having a conveyor wire and a conveyor felt wrapped about such cylinder. The suction cylinder contains several zones of different pressure including a pressing zone above which the pressure rolls are provided for a more effective water removal from the paper sheet and the conveyor felt. However, this patent does not show the use of a stationary suction couch device much less such a stationary couch device including a curved cover formed by ceramic cover strips separated by drainage slots or the use of a central cover strip of greater width which is engaged by a pressure roll pressing the paper sheet and conveyor together against the upper surface of such central cover strip at a position spaced from the drainage slots in the manner of the present invention.
U.S. Pat. No. 4,919,760 of Kerttula issued Apr. 24, 1990 shows another type of twin-wire papermaking machine including a forming shoe with a curved upper surface across which the conveyor wire is moved in order to form the pulp stock into a self-supporting sheet of paper which is then transmitted past flat suction boxes and a couch roll for greater water removal. However, there is no disclosure of a stationary couch device mounted on the papermaking machine at a position on the conveyor normally occupied by the rotating couch roll and employing a press roll in engagement with an enlarged central section of the curved cover of the stationary couch device in the manner of the present invention.
It has also been proposed to provide a stationary curved drainage device for dewatering the conveyor felt in the press section of the papermaking machine, as shown in U.S. Pat. No. 4,270,978 of Fioravanti issued June 2, 1981. However, there is no suggestion of using a pressure roll to urge the felt against the stationary curved drainage device for greater cleaning and dewatering efficiency in the manner of the present invention. It is conventional to provide a stationary flat suction box in the forming section of the papermaking machine for engagement with the conveyor wire as shown in U.S. Pat. No. 2,543,870 of Robbins, issued Mar. 6, 1951. However, there is no suggestion of employing a press roll to press the paper sheet and conveyor wire against the flat cover of such stationary suction box device.
The use of a pressure roll to press the conveyor wire and paper sheet against the perforated surface of a moving suction couch roll at high pressures on the order of approximately 200 pounds per lineal inch of pressure roll length, causes problems because the paper sheet protrudes through the openings in the couch roll causing uneven moisture distribution in the paper sheets and may result in hole patterns produced on the surface of the paper sheet and weakening of the sheet. This problem is avoided when the couch roll is a non-suction unperforated couch roll with no suction openings in its cover. However, even with an unperforated couch roll there is a problem due to water backup in front of the nip of the pressure roll where it presses the paper sheet and conveyor wire against the couch roll. In addition, water is thrown off the couch roll after the nip which can also produce an uneven moisture distribution in the paper sheet. These two problems are overcome by the stationary couch device of the present invention which employs a central unperforated cover strip of greater width than the other cover strips, which is engaged by the pressure roll. As a result there is no pushing of the paper sheet through the drainage slots between the ceramic strips on opposite sides of such central strip, and the water backup and water throw off conditions are eliminated by the dewatering of the nip due to the pressure within the stationary suction couch device.
The use of a pressure roll with slotted stationary suction box with cover strips of the same width and separated by drainage slots cause even greater non-uniform moisture distribution and marking of the paper sheet than the suction couch roll due to pressure of the pressure roll pushing the paper sheet and the conveyor wire into the slots. This would tend to cause higher loads on the drive mechanism for the conveyor wire, possibly damaging the conveyor wire. These problems are avoided in the present invention by using a central cover strip of greater width which is unperforated and contacting such central strip with the pressure roll without contacting the drainage slots on opposite sides thereof or the remaining narrow ceramic cover strips.
In addition to overcoming these problems, the present invention has the added advantage of providing the ability to run a lightly nipped pressure roll on a curved stationary couch cover of ceramic material thereby producing a compacted paper sheet having fewer fibrous lumps in the paper sheet. In addition, there is reduced moisture content as well as more uniform moisture distribution in the paper due to the use of a slotted cover for the stationary couch device of the present invention which removes water with greater efficiency than a suction couch roll. Also, there is no water backup in front of the nip or water throw off behind the nip, which tends to disturb the paper sheet.